511 



union between the fourth arch and ventral aorta, grooving the latter 

 vessel as shown in most of the figures. 



BB 



B 



D 



Fig. 2. Grai^hic reconstructions from pig embryos. A, 5,5 mm, Harvard Embryo- 

 logical Collection, 915. B, 6,0 mm, H. E. C, 918. C, 9,0 mm, H. E. C, 52. D, 

 10,0 mm, H. E. C, 402. D.Ao dorsal aorta. V.Ao ventral aorta. IV fourth aortic 

 arch. P. A pulmonary arch. P.IV fourth jihaiyngeal pouch. P.B postbranchial body. 



